Oysters and other bivalves, like mussels and clams, can live in the brackish waters of estuaries by adapting their behavior to the changing environment. During low tides when they are exposed to low-salinity water, oysters close up their shells and stop feeding. Isolated in their shells, oysters switch from aerobic respiration (breathing oxygen through their gills) to anaerobic respiration, which does not require oxygen. Many hours later, when the high tides return and the salinity and oxygen levels in the water are considerably higher, the oysters open their shells and return to feeding and breathing oxygen (Sumich, 1996). (Photo: Apalachicola NERRS site)